Ignition system employing saturable core reset by relaxation oscillator



June 1955 J. c. MORRISON ETAL 3, IGNITION SYSTEM EMPLOYING SATURABLE CORE RESET BY RELAXATION OSCILLATOR Filed Oct. so. 1961 INVENTORS,

JAMES C. MORRISON y LOUIS H. SEGALL mwdd ATTO EYS United States. Patent IGNITION SYSTEM EMPLOYING SATBLE CURE RESET BY RELAXATIGN GSQILLATOR James C. Morrison and Louis H. Sag-all, Sidney, N.Y.,

assignors to The Bendix Corporation, Sidney, N.Y., a

corporation of Deiaware Filed Oct. 30, 1961, Ser. No. 148,441 30 Claims. (Cl. 315209) This invention relates to an ignition system. In specific preferred embodiments thereof the invention relates to a breakerless ignition system for use, for example, with engines of the jet or ram jet type.

The invention has among its objects a provision of a novel improved circuit for converting steady direct current to direct current pulses.

Another object of the invention lies in the provision of a novel spark producing circuit, wherein the rate of spark production remains substantially constant over wide ranges of temperature and input voltage.

A further object of the invention lies in the provision of a direct current powered transistorized circuit which is capable of producing spark discharges of relatively high energy.

A still further object of the invention lies in the provision of a direct current powered ignition circuit which employs no breakers and which is capable of producing an igniting spark discharge of high intensity.

The above and further objects and novel features of the invention will more fully appear from the following description when the same is read in connection with the accompanying drawing. It is to be expressly understood, however, that the drawing is for the purpose of illustration only, and is not intended as a definition of the limits of the invention. 7

In the drawing, the sole figure is a wiring diagram of an ignition circuit in accordance with a preferred embodiment of the invention.

It is desirable, in engines of the jet or ram jet type, that the spark producing ignition circuit be capable of delivering sparks of high intensity to the igniter spark gap. It is also desirable that there be a continuously operating ignition system even after the engine has been started, to avoid the possibilities of flame-out of the engine during flight. Direct current powered ignition circuits have ordinarily required the use of a current interrupter such as a vibrator which is not adaptedtor continuous operation, particularly because of the wearing of the contacts.

The circuit of the present invention overcomes the above difliculties by the employment of a breakerless direct current interrupter, and thus is capable of continuous operation without damage. Further, the circuit of the invention, although employing transistors, overcomes the lack of high energy capacity which has been inherent in former transistorized ignition circuits.

Turning now to the drawing, the circuit is powered by a direct current source, which may range, for example, from 14-30 volts, such source being connected to the terminals 10 and 11. Negative terminal 11 is connected to ground, as shown. The positive terminal 10 is connected to a wire 12 to which there is connected a wire 13 leading to a relaxation oscillator. Interposed in Wire 13 is a resistor 14. Beyond such resistor, wire 13 is connected to a wire 15 which forms the end of a loop, in one branch of which there is interposed a potentiometer 16 and in the other branch of which there is interposed a resistor 17. The wire 18leading from potentiometer 16 connects to a wire 23 which, in turn, is connected to the emitter of a unijunction transistor 19. A wire 24 in the other branch of the loop, beyond resistor 17, is connected to the base two 22 of the transistor 13. The base one 21 of ice transistor 19 is connected by a wire 25 to a first end of a coil, 26 which is magnetically linked with a saturable magnetic core 29 which may be made, for example, of a material known as Delta-Max. The other end of coil 26 is connected to ground by a wire 27.

The variable center tap 36 of potentiometer 16 is connected by a wire 35 to a zener or breakdown diode 34, the other terminal of such diode being connected to ground through a wire 31 in which there is interposed a resistor 33'. A condenser 32 is connected in shunt between the junction of wires 18 and 23 and ground by a wire 30.

In an alternative, unillustrated embodiment of the in-- vention, a fixed resistor is substituted for potentiometer 16, wire 35, zener diode 34, and resistor 33' are omitted, and Wire connects the lower terminal of condenser 32 to ground. In a preferred embodiment of such alternative oscillator, the other principal parameters remain the same as those in the illustrated circuit; the resistor substituted for potentiometer 16 has a resistance of 1800 ohms. The repetition rate of such alternative embodiment of relaxation oscillator is determined by the resistance and capacitance in the emitter circuit of unijunction transistor 19. In a satisfactory circuit in accordance with such alternative embodiment of the invention, the principal parameters (other than that of the resistor substituted for potentiometer 16) of which will be given below, the repetition rate is 520 cycles per second, such rate remaining constant throughout the entire input voltage range. Transistor 19 becomes conductive or fires when the voltage differential between the emitter 20 and the base one 21 has reached the intrinsic stand-off ratio, that is, the value at which the transistor becomes conductive. Such ratio remains constant over wide ranges of temperature and input voltages, since as the input voltage changes, the voltage at point A changes, because the resistance of coil 26 and resistor 17 function as a voltage divider. Thus athigher voltages the voltage across the condenser 32 must rise to a higher value to cause conduction from the emitter 20 to the base one 21.

Stated in another manner, the rate of charging of condenser 32, in an oscillator circuit in accordance with the above-described unillustrated alternative circuit, is a function of the voltage applied to terminals 10 and 11. 'The ratio of the voltage of the emitter 20 to the voltage of base one 21 remains constant regardless of changes in the voltage applied to terminals 10 and 11. Therefore, the fre quency of oscillation of the alternative circuit, without the zener diode, remains constant despite changes in the voltage of the current supply for the oscillator.

It will be understood that in the ignition circuit shown, as the voltage between wires 69 and 82 increases, the output energy increases as the square of such voltage. This is undersirable, since, with an unduly high power output, erosion of the terminals of control gap 83 and of igniter gap 97 becomes serious. The illustrated preferred embodiment of circuit in accordance with the invention includes, in the oscillator portion thereof, means which controls the repetition rate of the oscillator so as to maintain the power output of the ignition circuit within desirable limits. 7

The zener diode .34 and resistor v33' form a circuit which is connected in parallel with the condenser 32 and the portion of the resistance of potentiometer .16 below adjustable contact 36. The function of zener diode 34 and resistor 33' is to cause the impedance across condenser 32 to vary somewhat as the input voltage changes. The result is that as voltage across wires 10 and 1 1 is increased, the shunt impedance across condenser 32 decreases and the time required to charge condenser 32 to that value of voltage which will cause unijunction thansistor 19 to become conductive or fire increases. It has been determined that, when the proper combination of zener voltage, resistance valve for resistor 33' and such portion of tap 36 have been selected, the repetition rate of the relaxation oscillator can be caused to vary approximately inversely as the square of theinput voltage, that is, the voltage between wires and 11. The total power output of the illustrated ignition circuit will therefore remain substantially constant despite variations in the voltage of the source of power connected to terminals '10 and 11.

The saturable core 2? forms a part of a saturable transformer 33. Also magnetically linked to the saturable magnetic core 29 is a primary Winding 37, one end of which is connected to a wire 3h. Wire 39 branches, as shown, one branch 40 being connected to wire 1-2. Interposed in wire 40 is a condenser 41. The other branch 42 is connected to a first end of a primary winding'43 of a power transformer 44. The other end of primary 43 is connected to wires 12 and 40. Power transformer 44 has a secondary winding 67, primary 43- and secondary 67 being magnetically linked to a magnetic core 66. The output of secondary .67 leads to an output section of the ignition circuit, tov be described.

A secondary winding 50 is magnetically linked with saturable core 29 of the saturable transformer 33. One end of secondary Stl'is connected 'by a wire 53 to the bases 55 and 56, respectively, of power transistors 49 and 57 by a wire 54. The other end of secondary 50 is connected by a wire 51 to the emitter 47 of transistor 49. Interposed inwir-e 51 is an emitter resistor 4-6. A wire 45, connected to the second end of primary 37, is connected to wire 51 and is connected to the emitter 59 of transistor 57. Interposed in wire 45 in advance of emitter "59 is an emitter resistor .61. The collectors 62 and 65 of transistors 49 and 57, respectively, are connected by a wire 64 which extends to ground.

iIt will be seen that the power transistors $9 and 57 are connected in parallel. It is preferred that the collectors 62 and 65 be grounded directly to the housing or heat sink, thereby gaining thephysical advantage of a common or grounded collector. The emitters 47 and 59 of the power transistors are connected both to the input signal and to the load, since the primary 37 and secondary 50 of transformer 33 are connected together at one end thereof by the wires 45 and 51, thereby gaining the electrical advantages, including high gain, of common emitter.

The secondary winding 67 of transformer 44 has lead wires 69 and '80 connected to its opposite ends. Connect ing wires 69 and '82 is a wire 71 which has a resistor 7-2 and a diode 74 interposed therein in series. Beyond wire 71 there is a second diode 76 interposed in wire 69. Wire 69 extends to a voltage step-up transformer 90 which is connected to an igniter gap 397 in a manner to be explained. A wire 82, connected to wire 7-1, extends to a first electrode of a control gap 83. The other electrode of such gap is connected to ground. Beyond diode 76, a wire 77 is shunted across wire 69 and wire 82. Interposed in 'wire 77 in series relationship are two condensers 79 and 81. Wire 80 connects one end of winding 67 with wire 77 between condensers '79 and 81. Beyond wire 77, a further wire 34 having a condenser 85 interposed therein is shunted across wires 69 and 82. A still further wire 86, connected between wires 69 and 8-2, has a resistor :87 interposed therein. A further resistor 90 is connected by a wire 89 from wire 69 to ground, as shown. p

p The thus described portion of the circuit from and including secondary Winding 67 of transformer 44 to the input of transformer 90 constitutes a voltage doubling system whereby the tank condenser made up of individual condensers 79, '81, and 85 is charged to a potential which is twice that existing across the ends of the secondary winding 67. When such tank condenser is charged to a predetermined voltage, the tank storage condenser partially discharges across control gap '83 into a loop which is formed by ground, wire '92, a condenser 94 interposed in such wire, the primary 91 of transformer 90, and the wire 69. The rush of current thus produced through primary winding 91 induces a high voltage in secondary winding 95 of transformer 90. Current from the secondary winding discharges through a wire 96 to the one electrode of the ignitergap 97, the other electrode of the gap being connected by a wire $9 to ground.

The operation of the relaxation oscillator has been explained above. Such oscillator functions with the intermediate portion of the circuit, including the saturable transformer 33, the transistors 49 and 57, and the power transformer 44 to produce charging pulses for the tank condenser as follows: The primary winding 43 of transformer 44, the primary winding 37 of transformer 33, and the parallel connected transistors 49 and 57 are connected in series. When power is first applied to the system, being assumed that the residual magnetism of the saturable core 29 of transformer 33 is Zero, leakage current through the parallel transistors 49 and 57 causes feedback to be induced in secondary winding 50 of transformer 33, thereby to render the transistors conductive. The current flow in such series circuit increases along a slope determined by the resistance and inductance of the series windings 37 and 43. For all practical purposes, the slope is determined by the transformer 44-, since the inductance and resistance of primary winding 37 of transformer 33 are negligible. Transformer 33will saturate when sufficient current flows through its primary Winding 37. At saturation, feedback is reduced to zero, and the transistors 49 and 57 are rendered non-conductive. When the core 29 of transformer 33 has become saturated and transistors 49 and 57 have been rendered non-conductive, no current other than leakage current flows until a resetting current flows in coil 26 of the relaxation oscillator, thereby erasing the residual magnetism of the saturable core 29.

It has been found that, with parameters having values on the order of those given below, the time required to complete one power cycle is 300 10 seconds. The total period is l/F or 1900 10 seconds. The ratio of time on to time ,olf is 300/1900 orapproximately 1:6.. This allows considerable time to dissipate the heat which is generated during the switching operations.

The resistors 46 and 61 insure that the transistors 49 and 57 accept equal portions of the load. 7 Further, such resistors provide temperature stabilization of the circuit. As a general rule, a resistance large enough to produce a one volt drop at maximum cur-rent will suiiice. Larger resistances will provide greater thermal stability, however.

It has been found that the provision of the condenser 41 connected across the primary winding 43 of transformer 44- markedly reduces the heating of the power transistors .49 and 57. Without such condenser in a typical circuit, switching off of the transistors occurred with the voltage at nearly 70 volts and the current at approximately 16.5 amperes. There is thus a peak switching power of 1155 watts. When, however, a condenser 41 of 10 mfd. is placed across the primary 43, when the transistor is switched off, the voltage transient or inductive spike occurs when the current is at substantially 4 amperes. The peak power at this point is now 280 Watts. Switching time remains substantially unchanged by the use of the condenser 41.

In a satisfactory circuit in accordance With the present invention, the following are values for the principal components. The saturable core transformer 33 had a core made of a-material known as Delta-Max 3T5515D2. Coil 26 was composed of 200 turns of No. 30 wire, coil 37 of 5 turns of No. 15 wire, and coil 50 of 50 turns of No. 23 wire. Resistors 46; and 61 each had a resistance of .16 ohm. Condenser 32 had a capacity of 1 mfd. Resistor 33' has a resistance of 500 ohms. Resistor 14 has a resistance of 47 ohms. Resistor 17 has a resistance of 330 ohms. Variable potentiometer 16 has a resistance in the range from 0-1000 ohms.

Although only a limited number of embodiments of the invention have been illustrated in the accompanying drawings and described in the foregoing specification, it is to be expressly understood that various changes, such as in the relative values of the parts, materials used, and the like, as well as in the suggested manner of use of the apparatus of the invention, may be made without depart- -direct current, the secondary winding of the transformer being connected in feedback relationship with the primary winding thereof and being connected to the transistor so as to render it conductive upon the induction of voltage in such secondary winding and to render it'non-conductive when the core of the transformer becomes saturated with residual magnetism, and means periodically to remove the residual magnetism of the saturated core of the transformer, whereby the transistor again becomes conductive upon the induction of voltage in the secondary winding of the transformer.

2. Electrical apparatus as claimed in claim 1, comprising a capacitor connected in parallel with the winding of the circuit device, whereby the voltage transient incident to the switching-off of the transistor occurs when the current is of a relatively small value.

3. Electrical apparatus as claimed in claim 1, wherein the means periodically to remove the residual magnetism of the core of the transformer comprises a relaxation oscillator including a coil magnetically coupled to the core of the transformer.

4. Electrical apparatus as claimed in claim 3, wherein the relaxation oscillator is powered by the direct current source, and includes a further transistor for switching the oscillator on and off.

5. Electrical apparatus as claimed in claim 4, comprising circuit means controlling the repetition rate of the relaxation oscillator so that such repetition rate varies inversely to variations in the voltage of the direct current source.

6. Electrical apparatus as claimed in claim 5, wherein the further transistor is a unijunction transistor having an emitter, a base one, and a base two, said emitter and base two being operatively connected to one terminal of said source and said base one being connected through said coil to the other terminal of said source, and wherein the circuit means controlling the repetition rate of the relaxa tion oscillator includes a zener diode which maintains the voltage applied to the emitter of the further transistor substantially constant independently of variations in the voltage of said source.

7. In an ignition circuit, a source of direct current power, a first, power transformer having a primary winding and a secondary winding, a second, saturable core transformer having a primary winding and a secondary winding, a plurality of similar parallel connected power transistors having emitters, the primary winding of the first transformer, the primary winding of the second transformer, and the parallel connected transistors being connected in series across the source of direct current, the secondary winding of the second transformer being connected in feedback relationship with the primary winding thereof and being connected to the transistors so as to render them conductive upon the induction of voltage in such secondary winding and to render them non-coductive when the core of the second transformer becomes saturated with residual magnetism, and means periodically to remove the residual magnetism of the saturated core of the second transformer, whereby the transistors again become conductive upon the induction of voltage in the secondary winding of the second transformer.

8. An ignition circuit as claimed in claim 7, comprising a resistor in series with the emitter of each transistor, the resistors having substantially the same resistance.

9. An ignition circuit as claimed in claim 7, comprising a capacitor connected in parallel with the primary winding of the first transformer, whereby to reduce the peak power at the switching-off period of the transistors.

14). An ignition circuit as claimed in claim 7, wherein the means periodically to remove the residual magnetism of the core of the second transformer comprises a relaxa tion oscillator including a coil magnetically coupled to the core of the second transformer.

' 11. An ignition circuit as claimed in claim 10, wherein the relaxation oscillator is powered by the direct current source, and includes a further transistor for switching the oscillator on and off.

12. An ignition circuit as claimed in claim 11, wherein the said further transistor is a unijunction transistor having an emitter, a base one, and a base two, base one being connected to one end of the coil, and comprising a loop having parallel first and second resistors connected, re spectively, to the emitter and base two of the further transistor.

13. An ignition circuit as claimed in claim 12, comprising a capacitor connected across the direct current source in series with the first resistor.

' 14. An ignition circuit as claimed in claim 13, comprising a voltage regulating circuit connected in shunt with the capacitor from an intermediate point of the first resistor, said last named circuit including a zener diode.

15. An ignition circuit as claimed in claim 14, wherein the voltage regulating circuit includes a further resistor in series with the zener diode.

16. In electrical apparatus, a source of direct current electrical energy, electronic valve means, means comprising an inductance connected to said source through said valve means, a saturable core transformer connected to said valve means for controlling the electrical conductivity thereof, and means including an oscillator for repetitively removing residual magnetism from the saturable core of said transformer.

17. Electrical apparatus as defined in claim 16 wherein the transformer is so connected with said valve means that the latter is rendered conductive when the core of said transformer is not saturated and non-conductive when said core becomes saturated.

18. In electrical apparatus, a source of direct current electrical energy, circuit means connected to be energized by said source, and means for repetitively interrupting and re-establishing the flow of current in said circuit means, said second-named means including a saturable core transformer having primary and secondary windings on a core and means powered by said source for repetitively resetting the core of the transformer from saturated to unsaturated condition to vary the frequency of said interruptions inversely to variations of the supply voltage of said source.

19. Electrical apparatus as defined in claim 18 wherein said last-named means comprises a relaxation oscillator.

20. Electrical apparatus as defined in claim 19 wherein said second-named means includes electronic valve means in said circuit means in series with the primary winding of the transformer and means inductively coupling the output of said oscillator to the core of said transformer.

21. Electrical apparatus as defined in claim 19 wherein said oscillator comprises an electronic valve, a triggering condenser for said valve, and a zener diode and a resistor connected in series across said condenser.

22. Electrical apparatus as defined in claim 21 wherein said electronic valve is a transistor.

23. In electrical apparatus, a source of electrical energy, means including an inductance winding connected to be energized by said source, normally non-conductive elecenemas tronic valve means for controlling the flow of said energy from said source through said inductance, means including a saturable core transformer for rendering saidvalve 'means alternately conductive and non-conductive, and an oscillator powered by said source and operative to reset I 26. Electrical apparatus as defined in claim 23 wherein' said oscillator comprises an output Winding inductively coupled -to the core of the transformer.

27. In an ignition system, an igniter gap, a storage condenser, control means for causing said condenser to discharge across said gap when the charge thereon attains a predetermined discharge value, means for charging said condenser step-by-step in increments to said discharge value, said last-named means including a source of direct current electrical energy, a charging circuit, and means for intermittently.interrupting the flow of current from said source through the charging circuit, said last-named means including means for varying the frequency of such interruptions inversely to variations in the voltage supplied by said source, whereby the discharge repetition rate of said condenser may be maintained relatively constant throughout a wide range of source voltages.

28. An ignition system as defined in claim 27 wherein said means for varying the frequency comprises electronic means responsive to variations in the voltage of said source.

29. An ignition system as defined in claim 27 wherein the means for varying the frequency includes an electronic oscillator and means for automatically varying the repetition rate of the oscillator inversely to variations in the source voltage.

30. In electrical apparatus, a source of direct current electrical energy, a load circuit connected to be energized by said source, means including electronic valve means in said load circuit for rendering the latter alternately conductive and non-conductive, and means powered by said source for controlling said first-named means including a normally non-conductive electronic valve, a triggering capacitor charged by said source for rendering said lastnamed valve conductive and a zener diode connected in shunt with said capacitor.

References Cited-by the Examiner UNITED STATES PATENTS 3,084,311 4/63 Culbertson 30788.5

OTHER REFERENCES GE notes on The Application of the Silicon Unijunction Transistor, T. P. Sylvan, pages 48 and 49, May 1961.

ARTHUR GAUSS, Primary Examiner.

GEORGE N. WESTBY, Examiner. 

27. IN AN IGNITION SYSTEM, AN IGNITER GAP, A STORAGE CONDENSER, CONTROL MEANS FOR CAUSING SAID CONDENSER TO DISCHARGE ACROSS SAID GAP WHEN THE CHARGE THEREON ATTAINS A PREDETERMINED DISCHARGE VALUE, MEANS FOR CHARGING SAID CONDENSER STEP-BY-STEP IN INCREMENTS TO SAID DISCHARGE VALUE, SAID LAST-NAMED MEANS INCLUDING A SOURCE OF DIRECT CURRENT ELECTRICAL ENERGY, A CHARGING CIRCUIT, AND MEANS FOR INTERMITTENTLY INTERRUPTING THE FLOW OF CURRENT FROM SAID SOURCE THROUGH THE CHARGING CIRUIT, SAID LAST-NAMED MEANS INCLUDING MEANS FOR VARYING THE FREQUENCY OF SUCH INTERRUPTIONS INVERSELY TO VARIATIONS IN THE VOLTAGE SUPPLIED BY SAID SOURCE, WHEREBY THE DISCHARGE REPETITION RATE OF SAID CONDENSER MAY BE MAINTAINED RELATIVELY CONSTANT THROUGHOUT A WIDE RANGE OF SOURCE VOLTAGES. 